Which of the following reagents can be used to distinguish between an aldehyde and a ketone?

The increasing order of the following compounds towards $HCN$ addition is:

Correct order of reactivity in nucleophilic addition reaction is:
$(i)$ Benzaldehyde
(ii) $p$-Methylbenzaldehyde
(iii) $p$-Nitrobenzaldehyde
(iv) Acetophenone

Metaformaldehyde is:

Consider the following chemical reaction.
$CH \equiv CH \xrightarrow[{(2) CO, HCl, AlCl_3}]{{(1) Red hot Fe tube, 873 K}} \text{Product}$
The number of $sp^{2}$ hybridized carbon atom$(s)$ present in the product is ..........

What products are formed on the reaction of $Propanal$ with $2-Methylpropanal$ in the presence of $NaOH$? Write the name of the reaction.